Integrated circuits, active and passive components, optical disk drives, and the like generate heat under use conditions that must be diffused to allow continuous use of the heat generating component. Heat sinks in the form of finned metal blocks and heat spreaders containing heat pipes are commonly attached to these heat generating components to allow excess heat to be conducted away and radiated into the atmosphere. Materials useful for providing a thermal bridge between the heat generating components and heat sinks/heat spreaders are known. Many of these materials are based on gel masses, liquid to solid phase change compounds, greases, or pads that must be mechanically clamped between a printed circuit board (PCB) and heat sink.
More recently, thermally conductive materials incorporating adhesives have been introduced. These thermally conductive adhesive materials typically form an adhesive bond between the heat generating component and heat sink/heat spreader so that no mechanical clamping is required. Both heat-activated (hot melt) and pressure sensitive adhesives have been used in thermally conductive adhesives. In all cases, these thermal interface materials need to be thermally enhanced (compared with unfilled or lightly filled polymer compositions), be dimensionally stable at elevated temperatures (heat generating components often run at 50° C. or higher), and be soft and conformable enough to provide good contact (wet-out) between the substrates. Typically, such thermally conductive adhesives have compromised thermal conductivity for softness/conformability or vice versa.
Articles incorporating a polymer foam core are characterized by the density of the foamed polymer being lower than the density of the pre-foamed polymeric matrix. The lowered density for the foam may be achieved in several known ways such as by foaming with chemical blowing agents or by interspersing microspheres within the matrix, the microspheres typically being made of glass or of certain polymeric materials, the former being detrimental to the softness/conformability of the foam.
Foams have been used to join two rigid substrates, or substrates with uneven or rough surfaces. However, heretofore, foams have not been used for thermal interface materials. It was believed that discontinuous voids in the thermal interface material should be avoided due to the insulating nature of such voids. Thus thermal conductivity was compromised.
In certain applications, a fire retardant feature may be needed and/or may be required by applicable regulations. For example, tapes to be used in electric or electronic applications may be directly exposed to electrical current, to short circuits, and/or to heat generated from the use of the associated electronic component or electrical device. Consequently, industry standards or regulations may impose conditions on the use of such tape articles that require qualifying tests be performed such as burn tests, and the like. For electrical and electronics applications, the industry standard flammability test is Underwriters Laboratories (UL 94 “Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances”).
In other applications, there is a need for rework/and or repair, such as for example, attaching an aluminum frame to a plasma display panel (PDP). In these applications, an easily removable attachment system such as a stretch-releasable attachment system would be beneficial.
Consequently, it is desirable to provide thermally conductive foams and thermally conductive adhesive interfaces that have acceptable thermal conductivity and are soft/conformable and methods for the manufacture of the thermal interface materials. It is also desirable to provide the foregoing thermally conductive articles in a fire retardant construction which optionally has stretch releasable properties.